The invention is generally concerned with adjusting the color of a light to a desired, or xe2x80x9ctarget,xe2x80x9d white. This is useful, for example, in spotlights for theatrical or film work. The invention is also concerned with adjusting the white of a liquid crystal display (xe2x80x9cLCDxe2x80x9d) to a target white. The invention is adaptable to any LCD system, whether projection or direct view.
Many different colors are specified as xe2x80x9cwhitexe2x80x9d in video and monitor systems. For example, 5400xc2x0 K is specified as white for cinema work, but 6500xc2x0 K is specified as white for video work. White colors form 3400xc2x0 K to 9300xc2x0 K are used for other applications. For an end user to accurately judge color of the finished product on a monitor, it is necessary to adjust the monitor white point to the white point specified for the particular application.
In CRT monitors, this can be accomplished by adjusting the relative gains of the red, green and blue video amplifiers so that when all three video signals (R, G an B) are equal, the desired target white is displayed. Changing the white point involves changing the relative gain of the red, green and blue amplifiers.
This type of system has several disadvantages when it is used on a LCD display. If the gain is done in the analog domain, it affects not only the white point, but, because of the non-linear nature of LCDs, all other colors as well. This then requires modified Gamma correction curves for each target white point. If the gain adjustment is done in the digital domain, it requires that some of the dynamic range be ignored and not used. Since 8 bits is only marginally enough bits for any serious color work, ignoring part of the dynamic range can lead to serious color artifacts. Even if the basic resolution is 10 or 12 bits, color artifacts can result from interpolating the video signal to a smaller dynamic range.
Therefore, it is desirable to be able to adjust the color of a LCD display (projection or direct view) without changing the input video signal in either the analog or digital domains. The adjustment should be optical and controllable while the projector is operating.
EP 909 974 covers the use of one or two retarders with a reflective LCD cell to make the system achromatic and improve the contrast. The retarder(s) are with the LCD between the polarization-sensitive beam splitter (xe2x80x9cPBSxe2x80x9d) and the mirror. The system of the invention puts the retarders in the illumination path so they have no effect on contrast. The system is not designed to minimize chromatic effects. Rather, the system of the invention is designed to add exactly the right amount of chromatic effects to allow color correction to target whites.
U.S. Pat. No. 5,875,008 describes the use of three sets of compensators, one with each LCD to adjust color point, efficiency and contrast of the individual LCDs. While one effect of this is to change the white point, that is not the goal; and the approach is different from the approach of the invention. Again, the compensators are not in the illumination path.
EP 390,511 describes the use of three sets of one or two compensators, one with each LCD to adjust color point, efficiency and contrast of the individual LCDs. While one effect of this is to change the white point, that is not the goal and the approach is different from the approach of the invention. Again, the compensators are not in the illumination path.
Kokai 07-294,869 describes a system that adjusts the polarizers of at least two LCDs to adjust the efficiency of the cell. This then adjusts the color balance and the white point. The system of the invention adjusts retarders, not polarizers, and one common retarder is used for all three colors. Adjusting polarizers involves a mechanical system. Adjusting retarders can be done electronically.
Application PHN17544 EP-P describes a method for adjustment of a desired white color of at least a portion of an image, by a system provided with a light source and an electro-optical modulator, wherein light from the light source is modulated by the electro-optical modulator into the image, wherein the system comprises with the electro-optical modulator cooperating, a light adjustment device, wherein the electro-optical modulator is driven for at least a portion to a desired maximum, the portion being imaged on the portion of the image, after which the color of said portion of the image is adjusted to the desired white color by the light adjustment device.
A device includes a cooperating light adjustment device for each of the respective LCD panels. Therefore, a two LCD panel projector comprises two light adjustment panels, and a three LCD panel projector comprises three light adjustment panels. Also an embodiment is described in which a stack of light adjustment devices is used, but still the number of light adjustment devices corresponds to the number of electro-optical modulators.
It is an object of the present invention to provide a system and a method for adjusting the color of a light to a target white.
It is an object of the present invention to provide a system and a method for adjusting the target white of a LCD system.
It is another object of the present invention to provide such a system that is adapted to either a projection or direct view LCD system.
It is an object of the present invention to provide a simple and reliable electronic system for adjusting the target white of a LCD system.
These objects and others that will become apparent from the following specification are achieved by a system for adjusting the color of a light source which comprises the following components:
a) a first polarizer to polarize the light source,
b) a fixed retarder with its optical axis at a first angle to the polarized light,
c) an electronically variable retarder, with its optical axis at a second angle to the polarized light, and
d) a second polarizer to polarize the resultant light.
It is preferred that the system consists essentially of components a) to d). It is also preferred that the components be present in the order a) to d). Alternatively, the components may be in the order a), c), b) and d). Preferably, the polarized light is partially polarized. The first polarizer to polarize the light source may comprise a polarization conversion system. Preferably, the fixed retarder is a quarter wave retarder. The variable retarder can vary in the range of 700-1100 nm. Preferably, the light source is used to illuminate a display with one or more spatial light modulators. It is preferred that the spatial light modulator(s) be liquid crystal device(s).
The invention comprises a system for color correction. One version of the system uses the partially polarized light produced by an Integrator/Polarization Conversion System (PCS) combination and an ECB cell and one or more fixed compensator foils. By adjusting the voltages on the ECB cell(s), it is possible to adjust the relative amounts of red, green and blue light reaching the LCDs and therefore adjust the color temperature of the projector. Since all adjustments are done electronically, not mechanically, the system is simple and reliable.
The color correction scheme requires that the basic colorimetry of the display system be correct for one white point. This approach then allows the white point color to be adjusted over sufficient range to allow correction to other white points. The system has been tested with the optics from the color drum projector, U.S. patent application Ser. No. 09/127,003 but would be adaptable to any LCD system, whether projection or direct view.